The idea in one sentence

Every metre of wire resists current a little. Current times resistance is the voltage you lose along the way, and that voltage never reaches the appliance.

A worked example

Take a single-phase 230 V circuit feeding a 10 A load 30 m away. The current travels out and back, so the loop is 60 m. Using the maximum resistance values for plain copper conductors at 20°C, raised by about 20 per cent for working temperature:

Many designers try to keep the drop to roughly 3 to 5 per cent depending on the circuit. By that yardstick, 1.5 sq mm on this run is marginal, while 2.5 sq mm is comfortable. The calculation ignores reactance and installation effects, so treat it as an illustration.

Where long runs hide in a house

• Garden and boundary lights
• A water pump far from the main board
• Terrace circuits, especially in multi-storey houses
• Detached garages and outbuildings
• Basement equipment

What a drop does to equipment

• Lights dim slightly
• Motors draw more current to compensate and run hotter
• Electronic equipment may reset or fail early
• Heating elements deliver less heat

What to do about it

1. Measure the real run length, including vertical sections
2. Calculate the drop or ask your electrician to
3. Step up one size where the run is long
4. Keep joints to a minimum, since each one adds resistance
5. Use a separate feeder for far-off loads instead of daisy-chaining

Use real numbers

Do not rely on the size printed on the wire alone. The conductor resistance should match the standard for that size. A thinner conductor sold as 2.5 sq mm will have higher resistance, and the voltage drop will be worse than your calculation predicted. A reputable maker lists the values in its data sheet. You can see how Botsoncab presents specifications on the TDS Downloads page, and multistrand flexible cables are among its listed products.

The cost of getting it wrong

Stepping up one wire size usually adds a modest amount to the cost of that circuit. Digging up a concealed run to replace it later costs far more. When in doubt on a long run, go one size bigger.

To learn how to separate real manufacturers from resellers, read the full guide on house wire manufacturers in India or send your circuit details to Botsoncab.